PLEASE HAND IN UNIVERSITY OF TORONTO Faculty of Arts and Science

Transcription

1 PLEASE HAND IN UNIVERSITY OF TORONTO Faculty of Arts and Science APRIL 2010 EXAMINATIONS CSC 108 H1S Instructors: Horton Duration 3 hours PLEASE HAND IN Examination Aids: None Student Number: Family Name(s): Given Name(s): Do not turn this page until you have received the signal to start. In the meantime, please read the instructions below carefully. # 1: / 7 # 2: / 6 This final examination paper consists of 10 questions on 20 pages (including this one). When you receive the signal to start, please make sure that your copy of the final examination is complete. Comments and docstrings are not required except where indicated, although they may help us mark your answers. They may also get you part marks if you can t figure out how to write the code. You do not need to put import statements in your answers. You may not use break or continue on this exam. If you use any space for rough work, indicate clearly what you want marked. Assume all input is valid unless otherwise indicated; there is no need to error-check. # 3: / 6 # 4: /12 # 5: / 6 # 6: /18 # 7: / 7 # 8: / 6 # 9: /10 # 10: / 8 TOTAL: /86 Page 1 of 20 Good Luck! cont d...

2 Question 1. Part (a) [7 marks] [3 marks] Write the following function, according to its docstring: def flip_lists(l): L is a list of 2-element lists. Reverse the order of the elements in the sublists of L. Part (b) [4 marks] Write the following function, according to its docstring: def valid_sizes(l, a, b): L is a list of lists; a and b are ints; a <= b. Return True iff each sublist has size between a and b inclusive. Page 2 of 20 Student #: cont d...

3 Question 2. [6 marks] Write the following function, according to its docstring: def interleave(s1, s2): Return a new string that contains the characters of string s1 and string s2 "interleaved" so that it contains the first character of s1, the first character of s2, the second character of s1, the second character of s2, etc. If one string is longer than the other, the "extra" characters are added on at the end. For example, interleave("ab", "12345") returns "a1b2345". Page 3 of 20 Student #: cont d...

4 Question 3. [6 marks] The left-hand column in the table below shows a series of code fragments to be interpreted by the Python shell. For each, show the expected output in the right-hand column; if it would generate an error say so, and give the reason why. Hint: Use the memory model to predict what will happen. The next page is provided for your rough work. Code x = (1, 2, 3) for item in x: item = item + 1 print x x = (1, 2, 3) for i in range(len(x)): x[i] = x[i] + 1 print x x = ([1, 2], [], [8, 9, 3]) for item in x: item.append(5) print x x = [6, 7, 8] for item in x: item = item + 1 print x x = [6, 7, 8] for i in range(len(x)): x[i] = x[i] + 1 print x x = [[1, 2], [], [8, 9, 3]] for item in x: item.append(5) print x Output (or error plus reason) Page 4 of 20 Student #: cont d...

5 Use the space below for rough work. This page will not be marked. Page 5 of 20 Student #: cont d...

6 Question 4. [12 marks] Below are several functions that attempt to return True iff the given string has two characters in a row that are the same. For each version, indicate whether or not the function works. If it does not, write a call to the function that demonstrates one case where it fails (i.e., the return value is incorrect or the function generates an error), and state what happens. Part (a) [3 marks] def doubles1(s): answer = False previous = s[0] for char in s: if char == previous: answer = True previous = char return answer Does the function work (circle one)?: works doesn t work If it doesn t work, a call to the function that demonstrates this: and the outcome of that call (circle one): incorrect return value error Part (b) [3 marks] def doubles2(s): if s == "": return False else: answer = False previous = s[0] for char in s: if char == previous: answer = True previous = char return answer Does the function work (circle one)?: works doesn t work If it doesn t work, a call to the function that demonstrates this: and the outcome of that call (circle one): incorrect return value error Page 6 of 20 Student #: cont d...

7 Part (c) [3 marks] def doubles3(s): answer = False if s == "": return False else: answer = False previous = s[0] i = 1 while i < len(s): if s[i] == previous: answer = True previous = s[i] i = i + 1 return answer Does the function work (circle one)?: works doesn t work If it doesn t work, a call to the function that demonstrates this: and the outcome of that call (circle one): incorrect return value error Part (d) [3 marks] def doubles4(s): answer = False i = 0 while i < len(s): if s[i] == s[i + 1]: answer = True i = i + 1 return answer Does the function work (circle one)?: works doesn t work If it doesn t work, a call to the function that demonstrates this: and the outcome of that call (circle one): incorrect return value error Page 7 of 20 Student #: cont d...

8 Question 5. [6 marks] Suppose we record information about students in a dictionary, where each key is student number (an int), and its value is a dictionary of information about that student. These inner dictionaries are structured like this: {"gpa": 3.95, "name": "Lucia", "campus": "stg"}. Suppose also that we have put the students into groups for a project, and that we keep track of the groups in a list of lists, where each inner list contains the student numbers of the members of one group. Write the following function according to its docstring: def describe_groups(groups, students): For each group, print the group size, average GPA, and the number of stg students. groups is a list, and students is a dict, as described above. Page 8 of 20 Student #: cont d...

9 Question 6. [18 marks] Students in my other course, csc343, get 3 grace days (sorry about that!). For each assignment, I have a file with the grades and information about grace day ussage. Here is the beginning of the Assignment 1 file: * CSC343H1S 20101: Assignment 1 marks for Horton s section A1 / 75 A1grace / ,Simpson,Homer,65, ,Kent,Clark,36, ,Horton,Diane,47, ,Flintstone,Wilma,70, ,Rubble,Barney,55,2 The first several lines are a header with information that is irrelevant to this question and will therefore need to be skipped over. The number of lines in the header is unknown; it ends with a blank line. The remaining lines have the following components, separated by commas: a nine-digit student number family name, which might include blanks first names, which might include blanks assignment grade (between 0 and 100) number of grade days used (between 0 and 3) The list of students may not be exactly the same from one assignment file to another, since students can add and drop courses. Your job is to write a program that will read my grades files a1.txt, a2.txt, and a3.txt and print the student number (one per line) of every student who has used more than three grace days in total. Your program will build and use a dictionary where each key is a student number, and its value is the number of grace days used by that student. The dictionary will start out empty, and each file you read will update it. You will write three helper functions and then the main block. Wherever appropriate, you should call functions from earlier parts of the question in your solutions for later parts, even if you haven t written them. Just assume they work. Page 9 of 20 Student #: cont d...

10 Part (a) [3 marks] Write this helper function, according to its docstring. def skip_header(r): r is an open reader for a grade file (in which nothing has been read yet). Skip past the header and the blank line, so that the next line to be read is the first line of student data. Part (b) [7 marks] Write this helper function, according to its docstring. def add_grace(r, grace_used): r is an open reader for a grade file (in which nothing has been read yet). grace_used is a dict whose keys are student numbers (as strings) and whose values are ints indicating the number of grace days used by a student. Read the grace day usage information in r and update grace_used for each student encountered in r. If a student is not in the dict, add him or her. Page 10 of 20 Student #: cont d...

11 Part (c) [3 marks] Write this helper function, according to its docstring. def print_overused(grace_used, limit): grace_used is a dict whose keys are student numbers (as strings) and whose values are ints indicating the number of grace days used by a student. Print, one per line, the student number of every student whose number of grace days used exceeds limit (an int). Part (d) [5 marks] Write the main block (including its if statement), so that the program accomplishes what was described in the question. Page 11 of 20 Student #: cont d...

12 Question 7. [7 marks] Below is the outline of a program for keeping track of your DVDs. Its main block will use a variable called dvds to store information about them. You have three tasks: 1. Complete the main loop so that it calls the appropriate function from the main_choices dictionary. Note that all these functions need one argument: dvds. 2. Modify the program as appropriate so that it initializes the variable dvds by loading it from a cpickle file called data.pkl, if that file exists. If not, it should initialize dvds to be an empty dictionary. Recall that you can use os.path.exists( data.pkl ) to see if file data.pkl exists. 3. Modify the program as so that right before it ends, it saves the contents of variable dvds in a cpickle file called data.pkl. You may add, delete or change existing code. If your new code won t fit in the space available, use arrows to point to where the pieces should go. import easygui, cpickle, sys, os # Definitions of functions acquire_dvd, list_dvds, lend_dvd and return_dvd # omitted to save space. def quit(dvds): sys.exit() if name == main : main_choices = {"lend" : lend_dvd, "return" : return_dvd, "acquire": acquire_dvd, "list": list_dvds, "quit": quit} while 1: choice = easygui.buttonbox("action", "DVD library", main_choices.keys()) # Make sure the user chose something. if choice: Page 12 of 20 Student #: cont d...

13 Question 8. [6 marks] Don t guess. There is a 1-mark deduction for wrong answers on this question. Part (a) [2 marks] def f1(l): sum = 0 for item in L: for i in range(1, 101): if item >= i: sum = sum + 1 return sum Let n be the size of the list L passed to this function. Which of the following most accurately describes how the runtime of this function grow as n grows? Circle one. (a) It grows linearly, like n does. (c) It grows less than linearly. (b) It grows quadratically, like n 2 does. (d) It grows more than quadratically. Part (b) [2 marks] def f2(l): sum = 0 i = 1 while i < len(l): sum = sum + L[i] i = i * 2 return sum Let n be the size of the list L passed to this function. Which of the following most accurately describes how the runtime of this function grow as n grows? Circle one. (a) It grows linearly, like n does. (c) It grows less than linearly. (b) It grows quadratically, like n 2 does. (d) It grows more than quadratically. Part (c) [2 marks] def f3(l): for i in range(len(l)): L[i] = L[i] + 1 sum = 0 for item in L: sum = sum + item return sum Let n be the size of the list L passed to this function. Which of the following most accurately describes how the runtime of this function grow as n grows? Circle one. (a) It grows linearly, like n does. (c) It grows less than linearly. (b) It grows quadratically, like n 2 does. (d) It grows more than quadratically. Page 13 of 20 Student #: cont d...

14 Question 9. [10 marks] Consider the following class: class AcademicHistory( builtin.object) A student s academic history. Methods defined here: cmp (self, other) Return -1 if this student s average is lower than other s, +1 if it is higher, and 0 if they are equal. init (self, stnum, family_name, first_name) A new academic history for the student with student number stnum (an int), and the given family_name and first_name (strs). str (self) Return a str with the student s student number, name, course results, and average grade. average(self) Return the average grade (a float) among this student s completed courses. complete(self, course, grade) Record the fact that the student completed this course (a str) with this grade (an int). Page 14 of 20 Student #: cont d...

15 Part (a) [7 marks] 1. Create an AcademicHistory for a student named Nora Kingston, and make variable nora refer to it. Record the fact that she completed CSC108 with a grade of Which method does the above code call without mentioning it by name? 3. Without calling any AcademicHistory method by name, print out Nora s academic history. 4. Suppose you have a second AcademicHistory called balraj. Write a loop that will record the fact that Balraj earned 100 in each of these courses: HIS150, MUS100 and POL Write a single line of code that will cause str to be called without mentioning it by name. Part (b) [3 marks] Complete the following function according to its docstring description. def best_student(l): L is a non-empty list of AcacemicHistories. Return the student number of the one with the highest average. Page 15 of 20 Student #: cont d...

16 Question 10. [8 marks] Throughout this question, assume that we are sorting lists into non-descending order. Do not guess. There is a one-mark deduction for incorrect answers. Part (a) [2 marks] Consider the following lists: L1 = [9, 8, 7, 6, 5, 4, 3, 2, 1] and L2 = [8, 1, 3, 6, 9, 7, 4, 2, 5] Which list would cause bubblesort to do more swaps? Circle one answer. L1 L2 they would both require an equal number of swaps Part (b) [2 marks] Suppose you have a list of 100 elements. Is it possible that if you call bubblesort on it, no items will move? Circle one. yes no Part (c) [2 marks] Consider a list of 100 distinct numbers in order from largest to smallest, in other words, backwards to the order we want. Which sorting technique would cause items to be moved more times, if called on this list? Circle one. selection sort insertion sort Part (d) [2 marks] We are partly through sorting a list, and have completed 4 passes through the data. The list currently contains [9, 12, 42, 59, 2, 99, 10, 60] Which sorting technique are we definitely not using. Circle one. selection sort insertion sort Page 16 of 20 Student #: cont d...

17 [Use the space below for rough work. This page will not be marked, unless you clearly indicate the part of your work that you want us to mark.] Page 17 of 20 Student #: cont d...

18 [Use the space below for rough work. This page will not be marked, unless you clearly indicate the part of your work that you want us to mark.] Page 18 of 20 Student #: cont d...

19 Short Python function/method descriptions: builtins : len(x) -> integer Return the length of the list, tuple, dict, or string x. max(l) -> value Return the largest value in L. min(l) -> value Return the smallest value in L. open(name[, mode]) -> file object Open a file. Legal modes are "r" (read), "w" (write), and "a" (append). range([start], stop, [step]) -> list of integers Return a list containing the integers starting with start and ending with stop - 1 with step specifying the amount to increment (or decrement). If start is not specified, the list starts at 0. If step is not specified, the values are incremented by 1. cpickle: dump(obj, file) Write an object in pickle format to the given file. load(file) --> object Load a pickle from the given file dict: D[k] --> value Return the value associated with the key k in D. k in d --> boolean Return True if k is a key in D and False otherwise. D.get(k) -> value Return D[k] if k in D, otherwise return None. D.keys() -> list of keys Return the keys of D. D.values() -> list of values Return the values associated with the keys of D. D.items() -> list of (key, value) pairs Return the (key, value) pairs of D, as 2-tuples. file (also called a "reader"): F.close() Close the file. F.read([size]) -> read at most size bytes, returned as a string. If the size argument is negative or omitted, read until EOF (End of File) is reached. F.readline([size]) -> next line from the file, as a string. Retain newline. A non-negative size argument limits the maximum number of bytes to return (an incomplete line may be returned then). Return an empty string at EOF. float: float(x) -> floating point number Convert a string or number to a floating point number, if possible. int: int(x) -> integer Convert a string or number to an integer, if possible. A floating point argument will be truncated towards zero. list: x in L --> boolean Return True if x is in L and False otherwise. L.append(x) Append x to the end of the list L. Page 19 of 20 Student #: cont d...

20 L.index(value) -> integer Returns the lowest index of value in L. L.insert(index, x) Insert x at position index. L.remove(value) Removes the first occurrence of value from L. L.reverse() Reverse *IN PLACE* L.sort() Sorts the list in ascending order. str: x in s --> boolean Return True if x is in s and False otherwise. str(x) -> string Convert an object into its string representation, if possible. S.find(sub[,i]) -> integer Return the lowest index in S (starting at S[i], if i is given) where the string sub is found or -1 if sub does not occur in S. S.index(sub) -> integer Like find but raises an exception if sub does not occur in S. S.isdigit() -> boolean Return True if all characters in S are digits and False otherwise. S.lower() -> string Return a copy of the string S converted to lowercase. S.lstrip([chars]) -> string Return a copy of the string S with leading whitespace removed. If chars is given and not None, remove characters in chars instead. S.replace(old, new) -> string Return a copy of string S with all occurrences of the string old replaced with the string new. S.rstrip([chars]) -> string Return a copy of the string S with trailing whitespace removed. If chars is given and not None, remove characters in chars instead. S.split([sep]) -> list of strings Return a list of the words in S, using string sep as the separator and any whitespace string if sep is not specified. S.strip() -> string Return a copy of S with leading and trailing whitespace removed. S.upper() -> string Return a copy of the string S converted to uppercase. Total Marks = 86 Page 20 of 20 Student #: End of Final Examination